Add {Response,DataSink}::is_alive

[ochafik]

This is to be able to detect if the client connection was lost.

Context: #1952 / ggerganov/llama.cpp#9679

As suggested by @ngxson, since is_writable / select_write may be a bit heavyweight, I tried using just select_read (given it's in a context when every other thread is done reading) but it didn't do the trick alone, so settled for is_socket_alive (which uses select_read + read_socket): seems to work.

Update: I finally realized having is_alive on DataSink isn't enough, so moved it to Response itself (by lack of a better place?). In the llama.cpp use case we need to test for liveness before status / headers are even written back. Added a tiny test to exercise this.

Tested (no new failures): cmake -B build -DHTTPLIB_TEST=1 -DHTTPLIB_REQUIRE_OPENSSL=0 -DHTTPLIB_COMPILE=1 -DHTTPLIB_TEST=1 -DHTTPLIB_REQUIRE_ZLIB=1 -DHTTPLIB_REQUIRE_BROTLI=1 && cmake --build build -j && ctest --test-dir build -j